In Roman mythology, Janus is a god with a head but with two opposite faces. By analogy, the term “Janus” qualifies any dissymmetric object, such as a spherical particle whereof the two hemispheres would be physically and/or chemically different.
In embodiments of the present invention, by the term Janus particles, is meant dissymmetric particles of micron or submicron size having two parts that are chemically different and/or have different polarities1,2. Due to these properties, these particles constitute a unique category of materials that have a growing interest to both the industry and the scientific community. In fact, such particles can be used in a large number of applications ranging from catalysis3 fields to therapeutic treatments4. Until now, most of the techniques and methods used to generate such objects required to break the symmetry by introducing an interface2,5,6,7. However, this has the disadvantage of making the preparation of large quantities of particles rather difficult in as far as most techniques usually lead to equivalents of a monolayer of materials, since the particle modifications take place in a two-dimensional reaction space.
As a consequence, there is thus an increasing need in the development of alternative techniques and methods for replacing the two-dimensional approaches with real three-dimensional techniques, which allow for an extrapolation (in the sense of a change of scale) of a small scale production of Janus particles (typically at a laboratory scale) towards a large-scale production of industrial type.
Currently, there are only three really specific three-dimensional methods, but which do not allow a fine adjusting of the drive force of the modification8,9,10. For example, one possible approach is based on the generation of charge carriers on semiconductors using light8 or antenna9 effects. Another interesting method is that described by Banin et al10 which consists in using the HAuCI4 compound to make a material grow on gold tubes or on cadmium selenide nanotubes.
Within this context, bipolar electrochemistry represents another attractive possibility of selectively modifying particles in a three-dimensional reaction medium. This concept, which was first described by Fleischmann et al11 in 1986, is based on the fact that when placing a conductive object in an electric field of high intensity between two electrodes, a polarization which is proportional to the electric field as well as to the characteristic dimensions of the object, appears. If the polarization is strong enough, the oxidation-reduction reactions can occur at the opposite ends of the object.
There are recent applications of this concept as a drive force in the electrochemiluminescence reactions12 as detection modes in capillary electrophoresis13, for the preparation of structured surfaces14, for the functionalization of membrane pores15, for the creation of electrical contacts16 and as a mechanism for moving micro-objects17.
The potential value V created between the two ends of a conductive substrate placed in an electric field is given by the equation (1) herebelow:V=Ed  (1)with E defining the overall electric field and d defining the size of the particle.
It results that when an electric field of appropriate intensity is used, the drive force which constitutes the potential difference V can be used to carry out oxidation reduction reactions at the two ends of the substrate, thus leading to dissymmetrization of the particles as is illustrated on FIG. 1 attached to the present application. On this figure, “+” indicates the oxidation site and “−” the reduction site.
In order to achieve the two oxidation-reduction reactions at the opposite sides of an object, the potential difference V must be in first approximation at least equal to the difference between the formal potentials of the two oxidation-reduction pairs involved. For example, if one wishes to carry out dissymmetric functionalization with gold at the negatively charged ends by means of tetrachloroaurate, the following reaction must be carried out:[AuIIICl4]−+3e−→Au0(s)+4Cl− E0=0.99 V vs NHE  (2)with NHE being the normal hydrogen electrode serving as reference.
In order to be able to balance the consumption of fillers, an oxidation reaction must take place at the opposite end assuming that it is consists in the oxidation of water:2H2O(l)→4H+(l)+O2(g)+4e− E0=1.23 V vs NHE  (3)
It immediately ensues that, in this case, a minimal potential difference of approximately:
                              Δ          ⁢                                          ⁢                      V            min                          =                                                            E                Au                0                                            Au                ⁢                                                                  ⁢                                  Cl                  4                  -                                                      -                                          E                                                      H                    2                                    ⁢                  O                                0                                            O                2                                              =                      0.24            ⁢                                                  ⁢            V                                              (        4        )            is required to trigger the reaction.
This becomes a problem inherent to this approach when the objects to be functionalized are of micro- or nanometric size, since E must then reach values of the order of MV m−1. This is not compatible with a conventional industrial environment, and particularly when using aqueous solutions, due to intrinsic parasitic reactions, which are accompanied by the formation of macroscopic gas bubbles at each electrode, such that it disrupts the orientation of objects in the electric field.
This problem was partly resolved by Bradley et al. using organic solvents, such as to enlarge the potential window of the electrolyte, and thereby making it possible to generate metal deposits dissymmetrically on different objects of micron or submicron size18, 19. However, the technique used by Bradley et al. has the disadvantage of requiring the need to immobilize the objects on a surface such as to prevent them from rotating, meaning that the technique developed by Bradley et al. is in fact still a two-dimensional method and not a real three dimensional method taking place in the entire volume of the reactor.
It has recently been demonstrated that it was possible to overcome these drawbacks by a method of capillary electrophoresis implemented such as to be able to apply a high electric field20,21. However, considering that the modification of the particles is carried out in a capillary whereof the internal diameter cannot exceed a few hundred microns, the production of Janus particles is very slow, making this method unprofitable for industrial application.